1. Field of the Invention
The present invention generally relates to a refrigerating system and a method of operating the same system. More particularly, the present invention relates to a method of operating a refrigerating system wherein at least a compressor, a heat-dissipation type heat exchanger, a throttling means and a heat-absorption type heat exchanger are connected in series to form a closed circuit, which includes a first refrigerant circuit section having a higher pressure and a second refrigerant circuit section having a lower evaporating pressure, so that the higher pressure in the closed circuit becomes the supercritical pressure of the refrigerant circulating in the closed circuit. Further, the present invention relates to a refrigerating system carrying out the said method. The refrigerating system and the method of operating the same system according to the present invention can be suitably used in an air-conditioner for an automobile.
2. Description of the Related Art
The refrigerating system disclosed in Japanese Unexamined Patent Publication (Kohyo) No. 6-510111 on the basis of PCT/NO91/00119, includes a compressor, a heat-dissipation type heat exchanger (gas cooler), a throttling means, a heat-absorption type heat exchanger (evaporator) and a vapor-liquid separator (accumulator), which are connected in series with each other to form a closed circuit, the refrigerating system being operated so that the higher pressure in the closed circuit becomes the supercritical pressure of the refrigerant circulating in the closed circuit. In this refrigerating system, the higher pressure is adjusted by detecting at least one operating condition such as the exit temperature of the gas cooler disposed on the higher pressure side as a heat-dissipation type heat exchanger and controlling the throttling means disposed downstream from the gas cooler in accordance with the detected operation condition(s), to minimize an energy consumption in the refrigerating system.
To minimize the energy consumption in a refrigerating system, the system should be operated under conditions wherein a coefficient of performance (COP=Q/W) becomes a maximum as defined by a ratio of a refrigerating performance (Q) of the evaporator to a compression work (w) applied to the compressor from outside. In this regard, as is apparent from the above equation, the value of COP is determined from both the refrigerating performance (Q) and the compression work (W). The larger the refrigerating performance (Q) of the evaporator; i.e., an enthalpy change of a refrigerant during the passage thereof through the evaporator (the difference in enthalpy between an exit of the evaporator and an entrance thereof); and the smaller the compression work (W) necessary for compressing the refrigerant in the compressor, the larger the above-mentioned value of COP.
In a refrigerating system operated under conditions where the higher pressure in the closed circuit constituting the refrigerating system becomes the supercritical pressure of refrigerant (such an system may properly be referred to as "a supercritical cycle refrigerating system" hereinafter), it is possible to increase the above-mentioned value of COP by increasing the higher pressure in the closed circuit constituting the refrigerating system and thereby increasing the above-mentioned refrigerating performance (Q), provided the refrigerant is maintained generally at a constant temperature at the exit of the gas cooler. Such a condition is never seen in a refrigerating system operating under conditions where both the higher pressure and the lower pressure are lower than the critical pressure of refrigerant (such an system may properly be referred to as "a subcritical cycle refrigerating system"). Accordingly, the action of the throttling means in the former is different from that in the subcritical cycle system.
In other words, as shown, in a pressure-enthalpy diagram of FIG. 7 which is a P-H diagram or Mollier diagram in a supercritical cycle using carbon dioxide (CO.sub.2) as a refrigerant, the refrigerating performance (Q) in the evaporator becomes larger as the difference (.DELTA.H.sub.1 =H.sub.A -H.sub.D) between enthalpy (H.sub.D) at the entrance (point D) of the evaporator and enthalpy (H.sub.A) at the exit (point A) thereof increases and as a mass flow rate of refrigerant circulating in the evaporator increases. When the degree of superheating becomes excessively larger at the exit of the evaporator (point A), the specific volume of refrigerant sucked into the compressor increases, and the volumetric efficiency of the compressor decreases, in accordance with the temperature increase of the discharged gas, which causes a reduction in the circulation rate of the refrigerant (the amount of refrigerant supplied to the evaporator within a unit time; kg/h), resulting in the deterioration of refrigerating performance (Q). To keep the degree of superheating at an approximately constant value and thereby to avoid the deterioration of refrigerating performance due to the reduction in the circulation rate of the refrigerant, it is necessary to maintain the enthalpy (H.sub.A) at the exit of the evaporator (point A) at an approximately constant value. The enthalpy (H.sub.D) at the entrance of the evaporator (point D) is equal to an enthalpy (H.sub.C) at an exit of the gas cooler (point C) because the expansion process is isenthalpic in the throttling means. Accordingly, it is possible to increase the difference (.DELTA.H.sub.1) between the enthalpy (H.sub.D) at the entrance of the gas cooler (point D) and the enthalpy (H.sub.A) at the exit of the evaporator (point A) and, therefore, the refrigerating performance (Q), by reducing the enthalpy (H.sub.C) at the exit of the gas cooler (point C). Since the higher pressure inside the gas cooler, wherein the refrigerant is under a supercritical pressure, is a single phase zone of high pressure vapor, the higher pressure is adjustable independently of the refrigerant temperature at the exit of the gas cooler (point C). If the refrigerant temperature is kept approximately constant at the exit of the gas cooler (point C) (for example, at 40.degree. C.; this temperature being approximately equal to that of the environmental air which exchanges heat with the refrigerant in the gas cooler), the enthalpy (H.sub.C) at the exit of the gas cooler (point C) is reduced as the higher pressure increases, as is apparent from an isothermal curve for 40.degree. C. shown in the P-H diagram of FIG. 7. Accordingly, it is possible to increase the above-mentioned refrigerating performance (Q=.DELTA.H.sub.1) and, therefore, the COP, by increasing the higher pressure to reduce the enthalpy (H.sub.C) at the exit of the gas cooler (point C), if the refrigerant temperature at the exit of the gas cooler (point C) is kept approximately constant.
On the other hand, if the higher pressure is increased while maintaining the refrigerant temperature at the exit of the gas cooler (point C) at an approximately constant value (for example, 40.degree. C.), the compression work necessary for the compressor (W=.DELTA.H.sub.2 =H.sub.B -H.sub.A) increases in accordance therewith. In this regard, an assumption is made that the compression in the compressor is adiabatic, the compression process is an isothermal change, and the compression work (w) is equal to the difference between the enthalpy (H.sub.A) at the entrance of the compressor (point A) and the enthalpy (H.sub.B) at the exit of the compressor (point B). Therefore, if the higher pressure becomes excessively high, the above-mentioned COP falls due to the increase in compression work (W).
From the facts stated above, there is an optimum value of the higher pressure under which the COP value, determined by the relationship between the refrigerating performance (Q) and the compression work (W), becomes a maximum when the refrigerant temperature at the exit of the gas cooler (point C) is at a certain value. If the optimum values of the higher pressures at various refrigerant temperatures at the exit of the gas cooler (point C) are obtained, an optimum control curve will be determined, as shown in FIG. 7.
In the supercritical cycle refrigerating system disclosed in the above-mentioned Japanese Unexamined Patent Publication (Kohyo) No. 6-510111, the refrigerant temperature and pressure are detected at the exit of the gas cooler (point C), and the optimum value of the higher pressure at the detected temperature is determined based on the above-mentioned optimum control curve. Thereafter, the throttling means is controlled in accordance with an actual higher pressure so that the actual pressure becomes the optimum pressure thus determined, whereby the COP value is maximized and the energy consumption in the refrigerating system is minimized.
In the automobile air conditioner wherein the rotation of an engine is used as an drive source for the compressor, there might be a case where, when the rotational speed of the engine increases, a power of the compressor also increases in accordance therewith, which in turn increases a circulation rate of refrigerant in the evaporator (kg/h) to excessively increase the refrigerating performance (Q). To avoid such excessive refrigeration due to the increase in the rotational speed, it is necessary to reduce the opening degree of the throttling means and thus decrease the circulation rate of the refrigerant. However, it is impossible to effectively prevent excessive refrigeration by merely reducing the opening degree of the throttling means, since the refrigerant temperature is lowered to a saturation temperature corresponding to a refrigerant pressure as the refrigerant pressure drops in the evaporator. Accordingly, when the engine rotational speed increases, not only must the opening degree of the throttling means be reduced, but also the discharge capacity of the compressor must be decreased. That is, if a variable displacement type compressor is employed, capable of varying a discharge capacity by detecting a suction pressure (a refrigerant pressure at the exit of the evaporator) or a refrigerant temperature at the exit of the evaporator, so that the discharge capacity of the compressor becomes smaller when the engine rotational speed increases, it must be expected that the refrigerant temperature increases in the evaporator due to the decrease in the refrigerant circulation rate and the increase in the suction pressure (i.e., the increase in the refrigerant pressure in the evaporator) due to the reduction in the discharge capacity, which can effectively prevent excessive refrigeration from occurring when the rotational speed increases.
However, the above-mentioned supercritical cycle refrigerating system has several problems. For example, when the discharge capacity of the compressor is modulated with the same control characteristic as that of the refrigerating system of subcritical cycle, it is difficult to quickly carry out the capacity control of the compressor when the engine rotational speed increases, because the action of the throttling means is different in the supercritical cycle from that in the subcritical cycle.
That is, according to the throttling means in the subcritical cycle refrigerating system, the refrigerant temperature is detected at the exit of the evaporator, and the optimum pressure corresponding to this detected temperature is compared with the actual refrigerant pressure at the exit of the evaporator to control the throttling means so that the actual refrigerant pressure at the exit of the evaporator becomes optimum. In this respect, the optimum pressure at the exit of the evaporator means a pressure under which the degree of superheating of the refrigerant is constant at the exit of the evaporator. More specifically, if the detected refrigerant temperature at the exit of the evaporator is, for example, 8.degree. C., an optimum pressure under which a constant degree of superheating (for example, 5.degree. C.) is obtained is defined (the saturation temperature corresponding to this optimum pressure is 3.degree. C.). Therefore, the circulation rate of the refrigerant through the evaporator is adjusted by controlling the opening degree of the throttling means so that the actual refrigerant pressure at the exit of the evaporator becomes the optimum pressure. In such a manner, it is possible to carry out the refrigerating operation, under the conditions at which the COP value becomes maximum, by controlling the opening degree of throttle means in accordance with the refrigerant temperature at the exit of the evaporator to adjust the refrigerant pressure at the exit of the evaporator so that the degree of superheating is maintained at a constant value.
When the engine rotational speed and, therefore, the rotational speed of a driving shaft of the compressor increases in the subcritical cycle refrigerating system wherein the throttling means operates in such a manner, the refrigerant is not completely vaporized in the evaporator due to the increase in the circulation rate of the refrigerant supplied to the evaporator from the compressor, and the refrigerant temperature is lowered at the exit of the evaporator in correspondence to the degree of superheating. If the refrigerant temperature is lowered at the exit of the evaporator, the optimum pressure is also lowered in accordance with the refrigerant temperature. Accordingly, the opening degree of the throttling means is reduced in order to lower the actual refrigerant pressure, at the exit of the evaporator, to the above-mentioned optimum pressure. Since the resistance against the refrigerant flow increases due to the throttling action of the throttling means, the circulation rate of the refrigerant through the evaporator is reduced. Also, since the refrigerant pressure in the evaporator is lowered, in accordance with the reduction in the circulation rate of the refrigerant, to lower the suction pressure of the compressor, the volumetric efficiency of the compressor deteriorates. Accordingly, due to the reduction in the circulation rate of the refrigerant in the evaporator and the deterioration of the volumetric efficiency of the compressor, the refrigerating performance is lowered to prevent excessive refrigeration. Further, since the suction pressure of the compressor and the refrigerant temperature at the exit of the evaporator are quickly lowered due to the throttling action of the throttling means, it is possible, by detecting such values, to quickly carry out the volumetric control of the compressor, which also prevents excessive refrigeration.
As stated above, in the subcritical cycle refrigerating system, since the throttling means quickly acts in the throttling direction, even if the rotational speed excessively increases, excessive refrigeration is assuredly prevented by reducing the circulation rate of the refrigerant and other measures. Also, since the throttling means acts in the throttling direction to quickly lower the suction pressure of the compressor, it is possible to quickly and assuredly carry out the volumetric control of the compressor by detecting such a suction pressure and other measures and, as a result, to prevent excessive refrigeration from occurring.
On the contrary, in the supercritical cycle refrigerating system, the maximization of COP and therefore the minimization of the energy consumption in the refrigerating system is achieved by adjusting the opening degree of the throttling means based on the detected refrigerant temperature and pressure at the exit of the gas cooler (point C), as stated above, so that the actual refrigerant pressure at the exit of the gas cooler (point C) becomes the optimum pressure at the detected temperature.
When the engine rotational speed and, therefore, the rotational speed of the driving shaft of the compressor increase in the refrigerating system of supercritical cycle in which the throttling means acts as described above, a mass flow rate of the refrigerant supplied to the gas cooler also increases, whereby a refrigerant pressure in the gas cooler (a higher pressure; a discharge pressure) becomes also higher. On the other hand, since the opening degree of the throttling means is adjusted so that the refrigerant pressure at the exit of the gas cooler is maintained at a constant value as stated above, the opening degree of the throttling means is made large to suppress the increase of the refrigerant pressure at the exit of the gas cooler. This causes a problem in that the action of the throttling means in the throttling direction lags to delay the adjustment of the refrigerating performance. Also, if the action of the throttling means in the throttling direction lags, the discharge pressure promptly increases, while the lowering of the suction pressure delays, which result in the delay of the volumetric control of the compressor based on the detection of the suction pressure or other measures and cause the delay of the adjustment of the refrigerating performance.